Bonding to Existing Concrete

posted by Bob

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.



AJ, the Fast Setting Cement Patcher can be placed from 2" down to 1/4" in thickness. For deeper areas you will need to do 2 layers. Give the first layer 30 minutes to dry and then apply the second layer. Be sure that the surface is properly cleaned and damp prior to application.
- Lee-Technical Service
Monday, July 25, 2016 at 2:50 PM

J. Holly, you can use Top-N-Bond to raise that area. It can be placed from 1/2" down to a featheredge in one layer. You can add additional layers after the previous layer has hardened, around and hour or two. Be sure that the surface is clean and free of any loose dust, debris, paints, sealers, and oils.
- Lee-Technical Service
Monday, July 25, 2016 at 2:27 PM

We have to cover an existing concrete manhole lid with a sidewalk. Due to the grades, 1-1/2" to 2-1/2" cover is all we can provide. Do you have a product(s) to use as a finish sidewalk, over a concrete slab, while being 1-1/2" thick that will hold up and not crack or crumble.
- AJ
Monday, July 25, 2016 at 10:45 AM

The edge of my drive actually crosses over the roadside curb. Can I build-up a concrete "transition" an inch or so thick down to 0" in the radius of the curb to eliminate the "bump" problem? We have damaged a couple of cars that went over that bump. If so, what do I need to do?
- J. Holly
Friday, July 22, 2016 at 5:07 PM

Gail, we do not have anything in our product line that will allow forklift and manufacturing traffic in a short period of time. You may want to consider looking into epoxy repair products. They go off really quick and can be open to traffic sooner than concrete.
- Lee-Technical Service
Thursday, July 21, 2016 at 10:32 AM

Hi, I have been assigned to repair for the cracks and damages on the floor of our production plant (roofing manufacturer) and we can't afford to shut down the plant, what product you can recommend to us so that we can repair this without stopping the operation and in just short time repair works.
- Gail
Wednesday, July 20, 2016 at 9:52 PM

Judy, the Flo-Coat will not work for that application. Sorry, but we do not have anything in our product line that will resurface marble.
- Lee-Technical Service
Monday, July 18, 2016 at 2:16 PM

Jerry, you can use the Top N Bond Concrete Patcher. This material can placed from 1/2" to a featheredge in one layer. Allow each layer an hour to harden. Check the surface and if it is hard place the next layer. Do not allow the material to set longer than 24 hours before applying the final layers.
- Lee-Technical Service
Monday, July 18, 2016 at 1:32 PM

I'm a DIY'er and I want to resurface an old marble vanity. I got sakrete flo-coat for the project. Got any tips or suggestions.
- Judy
Wednesday, July 13, 2016 at 11:56 AM

I have one area in my side walk that one section raised. It has been the same for about years. It has a 1.5 inch rise. Menards told me to use the plastic cement. Looked great, but now is breaking up as it did not bind to the existing concrete. I was able to just lift it all off. How can I fix it. It is a pain having the lift there when I blow snow.
- Jerry
Tuesday, July 12, 2016 at 3:18 PM



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